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<a name="Constant-expressions"></a>
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<hr>
<a name="Constant-expressions-1"></a>
<h4 class="subsection">11.6.1 Constant expressions</h4>
<a name="index-INTEGER_005fCST"></a>
<a name="index-tree_005fint_005fcst_005flt"></a>
<a name="index-tree_005fint_005fcst_005fequal"></a>
<a name="index-tree_005ffits_005fuhwi_005fp"></a>
<a name="index-tree_005ffits_005fshwi_005fp"></a>
<a name="index-tree_005fto_005fuhwi"></a>
<a name="index-tree_005fto_005fshwi"></a>
<a name="index-TREE_005fINT_005fCST_005fNUNITS"></a>
<a name="index-TREE_005fINT_005fCST_005fELT"></a>
<a name="index-TREE_005fINT_005fCST_005fLOW"></a>
<a name="index-REAL_005fCST"></a>
<a name="index-FIXED_005fCST"></a>
<a name="index-COMPLEX_005fCST"></a>
<a name="index-VECTOR_005fCST"></a>
<a name="index-STRING_005fCST"></a>
<a name="index-POLY_005fINT_005fCST"></a>
<a name="index-TREE_005fSTRING_005fLENGTH"></a>
<a name="index-TREE_005fSTRING_005fPOINTER"></a>

<p>The table below begins with constants, moves on to unary expressions,
then proceeds to binary expressions, and concludes with various other
kinds of expressions:
</p>
<dl compact="compact">
<dt><code>INTEGER_CST</code></dt>
<dd><p>These nodes represent integer constants.  Note that the type of these
constants is obtained with <code>TREE_TYPE</code>; they are not always of type
<code>int</code>.  In particular, <code>char</code> constants are represented with
<code>INTEGER_CST</code> nodes.  The value of the integer constant <code>e</code> is
represented in an array of HOST_WIDE_INT.   There are enough elements
in the array to represent the value without taking extra elements for
redundant 0s or -1.  The number of elements used to represent <code>e</code>
is available via <code>TREE_INT_CST_NUNITS</code>. Element <code>i</code> can be
extracted by using <code>TREE_INT_CST_ELT (e, i)</code>.
<code>TREE_INT_CST_LOW</code> is a shorthand for <code>TREE_INT_CST_ELT (e, 0)</code>.
</p>
<p>The functions <code>tree_fits_shwi_p</code> and <code>tree_fits_uhwi_p</code>
can be used to tell if the value is small enough to fit in a
signed HOST_WIDE_INT or an unsigned HOST_WIDE_INT respectively.
The value can then be extracted using <code>tree_to_shwi</code> and
<code>tree_to_uhwi</code>.
</p>
</dd>
<dt><code>REAL_CST</code></dt>
<dd>
<p>FIXME: Talk about how to obtain representations of this constant, do
comparisons, and so forth.
</p>
</dd>
<dt><code>FIXED_CST</code></dt>
<dd>
<p>These nodes represent fixed-point constants.  The type of these constants
is obtained with <code>TREE_TYPE</code>.  <code>TREE_FIXED_CST_PTR</code> points to
a <code>struct fixed_value</code>;  <code>TREE_FIXED_CST</code> returns the structure
itself.  <code>struct fixed_value</code> contains <code>data</code> with the size of two
<code>HOST_BITS_PER_WIDE_INT</code> and <code>mode</code> as the associated fixed-point
machine mode for <code>data</code>.
</p>
</dd>
<dt><code>COMPLEX_CST</code></dt>
<dd><p>These nodes are used to represent complex number constants, that is a
<code>__complex__</code> whose parts are constant nodes.  The
<code>TREE_REALPART</code> and <code>TREE_IMAGPART</code> return the real and the
imaginary parts respectively.
</p>
</dd>
<dt><code>VECTOR_CST</code></dt>
<dd><p>These nodes are used to represent vector constants.  Each vector
constant <var>v</var> is treated as a specific instance of an arbitrary-length
sequence that itself contains &lsquo;<samp>VECTOR_CST_NPATTERNS (<var>v</var>)</samp>&rsquo;
interleaved patterns.  Each pattern has the form:
</p>
<div class="smallexample">
<pre class="smallexample">{ <var>base0</var>, <var>base1</var>, <var>base1</var> + <var>step</var>, <var>base1</var> + <var>step</var> * 2, &hellip; }
</pre></div>

<p>The first three elements in each pattern are enough to determine the
values of the other elements.  However, if all <var>step</var>s are zero,
only the first two elements are needed.  If in addition each <var>base1</var>
is equal to the corresponding <var>base0</var>, only the first element in
each pattern is needed.  The number of encoded elements per pattern
is given by &lsquo;<samp>VECTOR_CST_NELTS_PER_PATTERN (<var>v</var>)</samp>&rsquo;.
</p>
<p>For example, the constant:
</p>
<div class="smallexample">
<pre class="smallexample">{ 0, 1, 2, 6, 3, 8, 4, 10, 5, 12, 6, 14, 7, 16, 8, 18 }
</pre></div>

<p>is interpreted as an interleaving of the sequences:
</p>
<div class="smallexample">
<pre class="smallexample">{ 0, 2, 3, 4, 5, 6, 7, 8 }
{ 1, 6, 8, 10, 12, 14, 16, 18 }
</pre></div>

<p>where the sequences are represented by the following patterns:
</p>
<div class="smallexample">
<pre class="smallexample"><var>base0</var> == 0, <var>base1</var> == 2, <var>step</var> == 1
<var>base0</var> == 1, <var>base1</var> == 6, <var>step</var> == 2
</pre></div>

<p>In this case:
</p>
<div class="smallexample">
<pre class="smallexample">VECTOR_CST_NPATTERNS (<var>v</var>) == 2
VECTOR_CST_NELTS_PER_PATTERN (<var>v</var>) == 3
</pre></div>

<p>The vector is therefore encoded using the first 6 elements
(&lsquo;<samp>{ 0, 1, 2, 6, 3, 8 }</samp>&rsquo;), with the remaining 10 elements
being implicit extensions of them.
</p>
<p>Sometimes this scheme can create two possible encodings of the same
vector.  For example { 0, 1 } could be seen as two patterns with
one element each or one pattern with two elements (<var>base0</var> and
<var>base1</var>).  The canonical encoding is always the one with the
fewest patterns or (if both encodings have the same number of
petterns) the one with the fewest encoded elements.
</p>
<p>&lsquo;<samp>vector_cst_encoding_nelts (<var>v</var>)</samp>&rsquo; gives the total number of
encoded elements in <var>v</var>, which is 6 in the example above.
<code>VECTOR_CST_ENCODED_ELTS (<var>v</var>)</code> gives a pointer to the elements
encoded in <var>v</var> and <code>VECTOR_CST_ENCODED_ELT (<var>v</var>, <var>i</var>)</code>
accesses the value of encoded element <var>i</var>.
</p>
<p>&lsquo;<samp>VECTOR_CST_DUPLICATE_P (<var>v</var>)</samp>&rsquo; is true if <var>v</var> simply contains
repeated instances of &lsquo;<samp>VECTOR_CST_NPATTERNS (<var>v</var>)</samp>&rsquo; values.  This is
a shorthand for testing &lsquo;<samp>VECTOR_CST_NELTS_PER_PATTERN (<var>v</var>) == 1</samp>&rsquo;.
</p>
<p>&lsquo;<samp>VECTOR_CST_STEPPED_P (<var>v</var>)</samp>&rsquo; is true if at least one
pattern in <var>v</var> has a nonzero step.  This is a shorthand for
testing &lsquo;<samp>VECTOR_CST_NELTS_PER_PATTERN (<var>v</var>) == 3</samp>&rsquo;.
</p>
<p>The utility function <code>vector_cst_elt</code> gives the value of an
arbitrary index as a <code>tree</code>.  <code>vector_cst_int_elt</code> gives
the same value as a <code>wide_int</code>.
</p>
</dd>
<dt><code>STRING_CST</code></dt>
<dd><p>These nodes represent string-constants.  The <code>TREE_STRING_LENGTH</code>
returns the length of the string, as an <code>int</code>.  The
<code>TREE_STRING_POINTER</code> is a <code>char*</code> containing the string
itself.  The string may not be <code>NUL</code>-terminated, and it may contain
embedded <code>NUL</code> characters.  Therefore, the
<code>TREE_STRING_LENGTH</code> includes the trailing <code>NUL</code> if it is
present.
</p>
<p>For wide string constants, the <code>TREE_STRING_LENGTH</code> is the number
of bytes in the string, and the <code>TREE_STRING_POINTER</code>
points to an array of the bytes of the string, as represented on the
target system (that is, as integers in the target endianness).  Wide and
non-wide string constants are distinguished only by the <code>TREE_TYPE</code>
of the <code>STRING_CST</code>.
</p>
<p>FIXME: The formats of string constants are not well-defined when the
target system bytes are not the same width as host system bytes.
</p>
</dd>
<dt><code>POLY_INT_CST</code></dt>
<dd><p>These nodes represent invariants that depend on some target-specific
runtime parameters.  They consist of <code>NUM_POLY_INT_COEFFS</code>
coefficients, with the first coefficient being the constant term and
the others being multipliers that are applied to the runtime parameters.
</p>
<p><code>POLY_INT_CST_ELT (<var>x</var>, <var>i</var>)</code> references coefficient number
<var>i</var> of <code>POLY_INT_CST</code> node <var>x</var>.  Each coefficient is an
<code>INTEGER_CST</code>.
</p>
</dd>
</dl>

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